A communication device to be used for wireless communication has a transmitter for transmitting a radio signal. The transmitter amplifies a modulated signal to be transmitted by means of an amplifier. The transmitter transmits the modulated signal amplified by the amplifier by radio through an antenna.
In a modulation system such as QPSK (Quadrature Phase Shift Keying), amplitude of the modulated signal temporally changes. The amplifier should preferably have a constant gain regardless of amplitude of a modulated signal inputted to the amplifier, in order to enhance reliability of the wireless communication. In order that the gain of the amplifier is made stable, a voltage value of power supplied to the amplifier is set according to maximum amplitude of the modulated signal to be inputted.
If the voltage of the power supplied to the amplifier is set according to the maximum amplitude of the modulated signal, the voltage of the power supplied to the amplifier remains high even if the amplitude of the modulated signal is small. If the amplitude of the modulated signal inputted to the amplifier is too small with respect to the voltage value of the power supplied to the amplifier, an amount of power loss of the amplifier grows high.
Change the voltage value of the power supplied to the amplifier according to the amplitude of the modulated signal inputted to the amplifier, so that the amount of power loss of the amplifier can be reduced. A method for changing the voltage of the power supplied to the amplifier on the track of a change of modulated amplitude with a frequency lower than the frequency of the modulated signal is called envelope tracking. Some technologies related to the envelope tracking according to a data rate of a transmitter are disclosed.
A technology related to the envelope tracking system is disclosed in Japanese Laid-open Patent Publication No. 06-77740 such that a transmitter extracts an envelope signal on the basis of a modulated signal outputted by a baseband processor. The extracted envelope signal is inputted to an output voltage control terminal of a DDC (DC DC Converter) which generates power supplied to an amplifier. As the DDC needs to respond to an amplitude change of the envelope signal, the disclosed system is fit for a communication system of a relatively low data rate around several hundred kHz such as a GSM (Global System for Mobile Communications) system.
A technology related to the envelope tracking is disclosed in Japanese Laid-open Patent Publication No. 2009-525684 such that a transmitter extracts an envelope signal from a modulated RF (Radio Frequency) signal. The transmitter of Japanese Laid-open Patent Publication No. 2009-525684 detects average power of the RF signal and extracts a difference between the detected signal and the RF signal to be transmitted so as to generate an envelope signal.
According to the envelope tracking system disclosed in Japanese Laid-open Patent Publication No. 2009-525684, a DDC produces a reference voltage of power supply to be applied to an amplifier on the basis of the average power of the detected RF signal, and superimposes the reference voltage of the power supply with the difference between the detected signal and the RF signal to be transmitted, as well. The reference voltage of the power supply is controlled in a closed loop including the amplifier. The closed loop includes an LPF (Low Pass Filter) for extracting the average power. A pass band of the LPF is optimized according to a frequency of the modulated signal. It is enough for the DDC of the disclosed system to follow the average power extracted by the LPF. The disclosed system is thereby compatible with a high data rate system of several tens MHz such as LTE (Long Term Evolution).
Suppose a transmitter to correspond to transmission of a plurality of modulated signals each having a different frequency. If the envelope tracking system of Japanese Laid-open Patent Publication No. 06-77740 is employed for such a transmitter, the DDC needs to correspond to an envelope signal of a high frequency. The higher frequency the DDC corresponds to, the lower conversion efficiency the DDC suffers, and thus causes power consumption efficiency of the entire transmitter to decrease. Meanwhile, if the envelope tracking system of Japanese Laid-open Patent Publication No. 2009-525684 is employed, it is necessary to optimize the pass band of the LPF according to a modulated signal of a high frequency. Then, the pass band of the LPF is made too broad in an operation according to a low bit rate communication system, and thus noise components can easily pass the LPF. As a result, the reference voltage value of the power supply outputted by the DDC is made unstable.